Apparatus and method for recording an information on a recordable optical record carrier using oval spot profile

ABSTRACT

A method and a corresponding apparatus for recording an information on a recordable optical record carrier ( 2 ) by irradiation of a light beam through an optical system ( 3 - 7 ) onto said record carrier ( 2 ) for forming marks and lands representing said information along an information recording direction (t), obtains a higher recording density, and thus higher data capacity, by using astigmatism for influencing the light beam from the light source ( 8 ) to the record carrier ( 2 ) during recording of information so as to obtain a light beam having a substantially oval spot profile having a shorter axis in the information recording direction (t), i.e., the tangential direction for an optical disc, compared to the direction (r) orthogonal thereto, i.e., the radial direction.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an apparatus for recording informationon a recordable optical record carrier by irradiation of a light beamonto said record carrier for forming marks and lands representing saidinformation along an information recording direction. The presentinvention relates further to a corresponding recording method, anoptical record carrier and a computer program for implementing saidmethod.

2. Description of the Related Art

The total data capacity of optical discs is determined by the radial andtangential data density. The radial density is determined by the datatrack pitch, the tangential data capacity by the shortest mark that canbe written. For rewritable phase-change discs, re-crystallization at thetrailing edge of a mark during writing of the next mark is used toobtain a mark of shorter length than the optical spot size. Typically,half of the mark is erased to end up with a crescent shaped mark. Forthe recently introduced Blu-ray Disc (BD), a total data capacity of 25Gbyte can be recorded on a single recording layer of a 12 cm disc. Theshortest length is 150 nm (d=1 code) while the optical spot is 300 nm inwidth (1/e radius of the optical spot is 150 nm).

A recordable (write-once) BD format is required in addition to the RW(rewritable) format. This so-called BD-R system should also achieve thehigh data-capacity of a BD RW system, i.e., 25 GB, to allow one-to-onecopies. In recordable media, mark formation occurs when a thresholdtemperature is exceeded. The mark-formation corresponds to anirreversible change in the information layer, and effects comparable tothe re-crystallization in rewritable media in principle do not occur.Thus, the marks that have been recorded in recordable media essentiallyreflect the optical/thermal profile that was present during therecording process. To reduce the mark length, thus to increase thetangential density, less laser power may be used to write a smallermark. As a consequence, the modulation of these smaller mark, and thussignal-to-noise ratio, will drop as well. This can be similarlydiscussed for write-once media based on phase-change material asrecording material. The as-deposited amorphous material willre-crystallize to form circularly shaped crystalline marks. Mark lengthreduction due to post heat is not possible.

U.S. Pat. No. 5,673,246 discloses an optical pick-up device which makesa numerical aperture of an object lens in a radial direction of anoptical disc smaller than that in a tangential direction to a track onthe optical disc. Thus, a spot diameter in the tangential direction tothe track becomes smaller than that in the radial direction duringread-out. Thus, it is possible to prevent degradation MTF (ModulationTransfer Function) as well as to accurately read data recorded on thedisc.

SUMMARY OF THE INVENTION

It is therefore an object of the present invention to provide arecording apparatus and a corresponding recording method for recordinginformation on a recordable optical record carrier by which datacapacities can be achieved that are similar to the data capacities ofcorresponding rewritable optical record carriers.

This object is achieved according to the present invention by arecording apparatus comprising:

a light source for generating a light beam, and

optical means for irradiating said light beam onto said record carrier,

wherein said optical means comprise means for influencing said lightbeam from said light source to said record carrier during recording ofinformation by use of astigmatism so as to obtain a light beam having asubstantial oval spot profile having a shorter axis in the informationrecording direction compared to the direction orthogonal thereto.

This object is further achieved according to the present invention by acorresponding recording method.

The present invention is based on the idea to use a non-circularspot-profile for the recording process. If the spot-profile is elongatedin the radial direction, an oval-like thermal profile on the recordinglayer is obtained, so that marks can be written that are narrow intangential direction compared to their radial extent thus increasing theresolution. Such an oval spot profile is obtained, according to thepresent invention, by use of astigmatic aberration of the wavefront ofthe light beam. As a result of astigmatism, two focal ‘lines’ (ovals)are formed. By choosing the proper direction of the astigmatism duringrecording, an oval spot profile with the long axis in the radialdirection is obtained. No optical power is thereby lost.

The proposed spot modification is also beneficial for high-speedwrite-once recording. At high recording speeds, thermal in-trackinterference may hamper the pit formation process. To reduce the thermalin-track interference, an ellipsoidal optical spot, such as obtainedwith the proposed modification, leads to less direct heating of thepreviously written pits.

According to a preferred embodiment, an astigmatism can be introducedinto the light beam deliberately, for instance, by use of liquid crystalmeans, such as a liquid crystal cell, or by use of a simple cylindricallens. Preferably, the liquid crystal cell is formed by a liquid crystalmaterial filling a cylindrical-lens-shaped cavity. When properlydesigned, the application of electrical voltage will cause the liquidcrystal molecules to change their orientation. This then results in achange of refractive index which allows the ‘lens’ to be turned on andoff. In the off-state, the refractive index of the liquid crystal shouldmatch that of the cell, so that no optical effect is introduced.

Preferably, the cell is oriented tangentially or radially compared tothe grooves on the disc. The best solution is to arrange the cell suchthat light in radial direction is affected by the liquid crystal cell.When the liquid crystal cell is off, light is focused on the discresulting in a (nearly) circular spot. If then the liquid crystal cellis switched on, rays in the radial direction get a little bit defocuseddue to the cylindrical lens resulting in the desired oval spot-shape,radially broadened, narrower in tangential direction. It is alsopossible to use a cell which affects the tangential direction, but whenit is then turned on, some re-focusing is required to get the desiredspot-shape.

According to an alternative embodiment, the focus position of the focallines of the light beam having an intrinsic astigmatism can becontrolled such that a defocus is introduced during recording ofinformation. Such an intrinsic astigmatism is generally produced by thelight source used, in particular, by a laser diode itself. For instance,edge-emitting laser diodes always have some intrinsic astigmatism due tothe internal shape of the device, i.e., the laser-mode is not fullyconfined to the optical gain region. This astigmatism only has theproper direction if the output of the laser is placed tangential orradial with respect to the grooves on the disc. The advantage is that noadditional elements are required in the recording device.

Finally, the proper direction of the spot on the disc is controlled bythe focus-offset. Both oval-directions can be obtained albeit atdifferent focal distances, e.g., different focus-offset. In an opticaldrive the disc is kept into focus by electronic means, i.e., the heightof the lens above the disc is controlled by actuators. The currentthrough the actuators is such as to minimize a so-called focus-errorsignal. Deliberate defocus can be introduced by adding, e.g., an offsetto the focus error signal. Such control means is present in existingdrives for the purpose of active focusing, this control means beingadapted, according to the present invention, for recording by use of anoval spot profile.

In addition, appropriate control means are provided for switching saidmeans for influencing the light beam on or off, or for bringing saidmeans into the light path during recording and for removing itthereafter, in particular, during read-out or RW-recording wherespot-deformation is not desired. For instance, a cylindrical lens can beeasily ‘turned on and off’ by mechanical placement of this lens in andout of the beam. Another possibility is to electrically switch a liquidcrystal cell on or off.

The proposed solution can also be helpful in high-speed recording. Thereason for this is that at high-speed recording, similar problems arefound as with high-density recording. In high-speed recording,subsequent marks are written with very little timing-delay because ofthe high speed. A consequence is that neighboring marks are affectingeach other via so-called thermal interference: heat present from theprevious (next) mark influences the formation of the next (previous)mark. By using a spot that is narrower in the tangential direction, sucheffects can be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be explained in more detail withreference to the drawings, in which:

FIGS. 1 a and 1 b show long and short marks recorded on rewritable andrecordable record carriers with a circular spot profile;

FIGS. 2 a and 2 b show long and short marks recorded with a circular andan oval spot profile, respectively, on recordable record carriers;

FIGS. 3 a and 3 b show an optical disc according to the presentinvention;

FIG. 4 illustrates the effect of astigmatism;

FIG. 5 shows a first embodiment of a recording apparatus according tothe present invention;

FIG. 6 shows a second embodiment of a recording apparatus according tothe present invention;

FIG. 7 shows a third embodiment of a recording apparatus according tothe present invention;

FIG. 8 shows the optical spot profile used for read-out of data; and

FIG. 9 shows the optical spot profile used for recording of data.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

To achieve data capacities in write-once systems that are similar to thedata capacities of corresponding rewritable systems, very narrow (intangential direction t) marks need to be written at short run-lengths,while in rewritable media, the short marks are as broad (in radialdirection r) as the long marks. In recordable media, the shortest marksare as broad as their lengths, i.e., have a circular shape. Thus, thewidth of the shortest marks decreases resulting in lower modulationduring read-out. This is illustrated in FIGS. 1 a and 1 b showing aschematic drawing of long (8T) and short (2T) marks recorded inrewritable (RW) media (FIG. 1 a) and recordable (R) media (FIG. 1 b) athigh density. It should be noted that the shortest marks are comparableor smaller than the optical spot profile. The consequence is that duringread-out, the optical modulation of the shortest marks in recordablemedia vanishes.

This problem can also be illustrated as well by comparing the resolution(=2T_(pp)/8T_(pp)) of the 2T marks for BD-RW and BD-R at a density of 23GB. According to the BD-RW specification, the 2T resolution should belarger than 10%, and this is met in practice. However, so far even inthe best BD-R media, the resolution at 23 GB is at most 5%. This poorresolution significantly deteriorates the jitter. The current inventionprovides a solution to write small but broad marks, i.e., having a hightangential density, such that a high modulation is preserved.

FIGS. 2 a and 2 b show a schematic drawing of long (8T) and short (2T)marks both recorded on a recordable medium. The marks shown in FIG. 2 ahave been recorded using a circular spot shown on the right-hand side,while the marks shown in FIG. 2 b have been recorded using an oval spotshown at the right-hand side, said oval spot having a shorter axis inthe tangential direction t compared to the axis in the radial directionr. As can be seen, the marks shown in FIG. 2 b are narrow in thetangential direction t compared to their radial extent and compared tothe marks shown in FIG. 2 a. Thus, the desired increase of theresolution can be obtained.

FIGS. 3 a and 3 b show a recordable optical disc according to thepresent invention in a top-view (FIG. 3 a) and in a cross-sectional view(FIG. 3 b). The disc 2 comprises a number of spiral grooves 13, whichindicate the direction in which the information is recorded on the disc2, i.e., the information is recorded along said spiral grooves 13. Thetangential direction t and the radial direction r are further indicatedin FIG. 3 a.

FIG. 3 b shows, in a cross-sectional view, the different layers of thedisc 2 which, in general, include a substrate layer 2 a, a recordinglayer or recording stack 2 b and a cover layer 2 c.

FIG. 4 illustrates the effect of astigmatism. In it's simplest form,astigmatism means that for a focused beam of light traveling in thez-direction, the focal distance of the beam is different in twoperpendicular directions, e.g., the x- and y-directions. Astigmatism canbe achieved by putting a cylindrical lens C in the light-path L. Thiscylindrical lens C causes light in one direction, for instance x, to bebended, but does not affect the light in the y-direction. If a normal(spherical) lens S, e.g., the objective lens of an optical recorder, isplaced behind this cylindrical lens C, the result will be that light inthe x-direction will be focused first, which was already somewhat bendedby the cylindrical lens C, while the y-direction is not yet fullyfocused. This will result in a line-shaped (or oval) focus Ox. At asomewhat further distance, the y-direction will be in complete focusbut, at the same time, the x-direction is already beyond focus resultingagain in a line-shaped (oval) focus Oy perpendicular to the first one.

A first embodiment of a recording apparatus according to the presentinvention is schematically shown in FIG. 5. During recording orread-out, the disc 2 is rotated by rotating means 1. A light beamgenerated by a light source, in particular, a laser diode 8, is focusedonto the disc by an objective lens 3 after the light beam has passed acollimator lens 7, a liquid crystal cell 14, a polarizing beam splitter6, a quarter-wave plate 5 and a diaphragm 4 for aperture limitation.During read-out, the polarizing beam splitter 6 reflects light reflectedfrom the disc 2 through a lens 9 onto a photo detector (array) 10 foranalyzing the reflected light. The general function of such a recordingapparatus is widely known and shall therefore not be explained furtherhere. Switching of the liquid crystal cell 4 can be done electrically.

The liquid crystal cell is formed by a liquid crystal material filling acylindrical-lens-shaped cavity. When properly designed, the applicationof electrical voltage to the cell will cause the liquid crystalmolecules to change their orientation. This then results in a change ofrefractive index which allows the ‘lens’ to be turned on and off. In theoff-state, the refractive index of the liquid crystal should match thatof the cell, so that no optical effect is introduced. Preferably, theliquid crystal cell is radially oriented so that light in the radialdirection is affected.

Instead of the liquid crystal cell 14, a cylindrical lens may bealternatively placed in the light beam as well, in which the cylindricallens is mechanically positioned in and out of the light beam. Further,either the liquid crystal cell 14 or the cylindrical lens can also bepositioned between the quarter-wave plate 5 and the polarizing beamsplitter 6 as shown in FIG. 6.

FIG. 7 shows another embodiment of a recording apparatus according tothe present invention. Therein, focus control means 15 is provided forcontrolling the height of the objective lens 3 above the disc 2, e.g.,by appropriate actuators (not shown). The current through the actuatorsis controlled so that a so-called focus-error signal is minimized.

In this embodiment, use is made of the intrinsic astigmatism which isgenerally produced by the light source 8 used, in particular, by a laserdiode itself. In case of recording, the focus control means 15 can be(partially) disabled, for instance, electrically or mechanically, or adefocus can be deliberately introduced by adding, e.g., an offset to thefocus error signal during recording of information by an offset settingmeans 16 which can be turned on and off.

The focused optical spot profile and marks/pits obtained by therecording method according to the present invention in a groove G (13)separated from another groove G by a land area L are shown in FIG. 8.During reading along the information recording direction t (=tangentialdirection), a circular optical spot profile 11 a is used. Shown are alsothe recorded marks 12 having an oval profile.

FIG. 9 shows the optical spot profile 11 b used for recording of data.As can be seen, an oval spot profile 11 b is used resulting in ovalmarks 12.

The present invention provides a solution to obtain higher datacapacities when recording in particular on recordable optical recordcarriers. Astigmatism is used for influencing the light beam from thelight source to the record carrier during recording of informationaccording to the present invention to obtain a light beam having asubstantially oval spot profile during recording, in particular, havinga smaller width in the tangential direction compared to the width inradial direction.

1. A recording apparatus for recording information on a recordableoptical record carrier by irradiation of a light beam onto said recordcarrier for forming marks and lands representing said information alongan information recording direction, comprising: a light source forgenerating a light beam; and optical means for irradiating said lightbeam onto said record carrier, wherein said optical means comprisesmeans for influencing said light beam from said light source to saidrecord carrier during recording of information, said influencing meansusing astigmatism to obtain a light beam having a substantial oval spotprofile having a shorter axis in the information recording directioncompared to a radial direction orthogonal to the information recordingdirection, and wherein said recording apparatus further comprisescontrol means for controlling said means for influencing the light beamby switching said means on or off by positioning said means forinfluencing into the light path during recording.
 2. The recordingapparatus as claimed in claim 1, wherein said means for influencing thelight beam introduces astigmatism into the light beam.
 3. The recordingapparatus as claimed in claim 2, wherein said means for influencing thelight beam comprises a liquid crystal cell.
 4. The recording apparatusas claimed in claim 3, wherein said liquid crystal cell has acylindrical shape.
 5. The recording apparatus as claimed in claim 2,wherein said means for influencing the light beam comprises acylindrical lens.
 6. The recording apparatus as claimed in claim 1,wherein said means for influencing the light beam comprises focuscontrol means for controlling a focus position of focal lines of thelight beam, said light beam having an intrinsic astigmatism, such that adefocus is introduced during recording of information.
 7. The recordingapparatus as claimed in claim 6, wherein said focus control means addsan offset to a focus error signal used for keeping the light beam intofocus during recording of information.
 8. A method of recording aninformation on a recordable optical record carrier by irradiation of alight beam through optical means onto said record carrier for formingmarks and lands representing said information along an informationrecording direction, said method comprising the steps of: generating alight beam; irradiating said optical record carrier with said lightbeam, including, during recording of information, influencing said lightbeam, through the use of astigmatism, to have a substantially oval spotprofile having a shorter axis in the information recording directioncompared to a radial direction orthogonal to the information recordingdirection; and controlling said influencing of the light beam byswitching said influencing on or off by positioning means forinfluencing into the light path during recording.
 9. A computer-readablemedium encoded with a computer program comprising instructions forcausing a computer to perform the steps of the method as claimed inclaim 8.